The invention relates to a process for preparing heat cured rubber (HCR) compositions.
An HCR composition comprises a high viscosity silicone polymer, an inorganic filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent or catalyst can be added and the composition heat cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the HCR composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler, treating agents and additives are batch mixed until desired properties are obtained. In Kasahara et al., U.S. Pat. No. 5,198,171, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed by a high speed mechanical shearing mixer. The resulting premix is further compounded in a same-direction double screw extruder. The premix is formed in a first step wherein a diorganopolysiloxane having a viscosity at 25xc2x0 C. of 1xc3x97105 cP or more, an inorganic filler and a treating agent are mixed in a high speed mechanical shearing machine to provide a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The flowable particulate mixture is then fed at a constant feed rate into a kneading and extruding machine that has two screws rotating in the same direction.
A batch process requires long mixing times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
In Hamada et al., U.S. Pat. No. 5,409,978, a preconcentrate of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200xc2x0 C. to 300xc2x0 C. in a co-rotating continuous double screw extruder. The preconcentrate is then compounded and heat treated at 150xc2x0 C. to 300xc2x0 C. in a counter-rotating, double screw extruder.
In a batch process, the silicone polymer, filler and treating agents can be accurately measured and charged into a mixer. However in a process such as the Hamada et al. process, polymer, filler and treating agent must be continuously fed as streams to the compounding apparatus. Control of continuously fed streams of materials is difficult but very important since component variations result in property variations in the compounded finished product. There is a need for a continuous process that can be controlled to provide products with consistent properties.
The invention provides a process that controllably compounds filler, processing fluid and silicone polymer into homogeneous filled HCR compositions with requisite reinforcing properties and levels of volatiles. The process comprises monitoring a continuous feed of a silicone polymer to a compounding apparatus and generating a signal in accordance with the monitored rate of feed. Rate of feed of filler and additives to the compounding apparatus are then adjusted in accordance with the signal.
In an embodiment of the invention, a silicone polymer is fed by means of a weighing belt to a compounding apparatus. The silicone polymer is compounded in the apparatus with filler and additives to form an HCR composition.
In another embodiment, compounding of an HCR composition is controlled by steps of separately sensing each of a rate of feed of silicone polymer, rate of feed of filler and rate of feed of additive to a compounding apparatus. Each of the sensed rates of feed is compared to a model of rates of feed for compounding a target HCR composition and at least one of the rates of feed is adjusted according to the comparing step.
In still another embodiment, an apparatus for compounding an HCR composition comprises a controllable silicone polymer feeder and controllable filler feeder, a signal generator associated with each of the silicone polymer feeder and the filler feeder to produce a signal proportionate to a feed rate of polymer and a signal proportionate to a feed rate of filler and a controller to receive the signals, to compare the signals to a model of feed rates and to emit a signal to adjust the feed rate of polymer, filler or both according to the comparison.